Why is the ground state of m-benzoquinone a triplet?

Abstract

The electronic structure, changes in aromatic behavior, and stability of the m-quinone, the least studied member of the benzoquinone family, have been analyzed. Through unrestricted density functional theory calculations, three aromaticity criteria (magnetic, electronic, and geometric), and resonance forms through the valence bond model, we have sought to answer why this isomer prefers the triplet state in its ground state. The answer is given in terms of spin density delocalization and proposed resonance structure contributions. The triplet state presents a higher spin delocalization which can be observed in the condensed values and the higher number of resonance forms contributing to the delocalization. Additionally, using magnetic criteria, we have used a simple but effective model to quantify the contribution of the proposed resonance structures, which have an excellent agreement with the spin density values.